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HFS+, or Hierarchical File System Plus, is the file system designed by Apple Computer[http://www.apple.com] to supersede HFS. First introduced with Mac OS 8.1, one of the biggest differences was the lower allocation block size of 4kb, which increased performance and lowered fragmentation (See:  Technical Note TN1121). It also implemented Unicode (rather than Mac proprietary formats) for naming files.
 
  
There are structurally many differences between HFS and HFS+, which are listed below[http://developer.apple.com/technotes/tn/tn1150.html#HFSPlusBasics]:
 
<br><br>
 
<CENTER><TABLE Border=1 cellpadding=2 cellspacing=0 width=75%>
 
            <TR>
 
              <TD>
 
                  <P><B>Feature</B></p>
 
 
              </TD><TD>
 
                  <P><B>HFS</B></p>
 
              </TD><TD>
 
                  <P><B>HFS Plus</B></p>
 
              </TD><TD>
 
                  <P><B>Benefit/Comment</B></p>
 
              </TD></TR>
 
 
            <TR>
 
              <TD>
 
                  <P>User visible name</p>
 
              </TD><TD>
 
                  <P>Mac OS Standard</p>
 
              </TD><TD>
 
                  <P>Mac OS Extended</p>
 
 
              </TD><TD>
 
                  <P></p>
 
              </TD></TR>
 
            <TR>
 
              <TD>
 
                  <P>Number of allocation blocks</p>
 
              </TD><TD>
 
                  <P>16 bits worth</p>
 
 
              </TD><TD>
 
                  <P>32 bits worth</p>
 
              </TD><TD>
 
                  <P>Radical decrease in disk space used on large
 
                  volumes, and a larger number of files per volume.</p>
 
              </TD></TR>
 
            <TR>
 
              <TD>
 
                  <P>Long file names</p>
 
 
              </TD><TD>
 
                  <P>31 characters</p>
 
              </TD><TD>
 
                  <P>255 characters</p>
 
              </TD><TD>
 
                  <P>Obvious user benefit; also improves
 
                  cross-platform compatibility</p>
 
              </TD></TR>
 
 
            <TR>
 
              <TD>
 
                  <P>File name encoding</p>
 
              </TD><TD>
 
                  <P>MacRoman</p>
 
              </TD><TD>
 
                  <P>Unicode</p>
 
 
              </TD><TD>
 
                  <P>Allows for international-friendly file names,
 
                  including mixed script names</p>
 
              </TD></TR>
 
            <TR>
 
              <TD>
 
                  <P>File/folder attributes</p>
 
              </TD><TD>
 
                  <P>Support for fixed size attributes (FileInfo and
 
                  ExtendedFileInfo)</p>
 
 
              </TD><TD>
 
                  <P>Allows for future meta-data extensions</p>
 
              </TD><TD>
 
                  <P>Future systems may use metadata for a richer
 
                  Finder experience</p>
 
              </TD></TR>
 
            <TR>
 
              <TD>
 
                  <P>OS startup support</p>
 
 
              </TD><TD>
 
                  <P>System Folder ID</p>
 
              </TD><TD>
 
                  <P>Also supports a dedicated startup file</p>
 
              </TD><TD>
 
                  <P>May help non-Mac OS systems to boot from HFS
 
                  Plus volumes</p>
 
              </TD></TR>
 
 
            <TR>
 
              <TD>
 
                  <P>catalog node size</p>
 
              </TD><TD>
 
                  <P>512 bytes</p>
 
              </TD><TD>
 
                  <P>4 KB</p>
 
 
              </TD><TD>
 
                  <P>Maintains efficiency in the face of the other
 
                  changes. (This larger catalog node size is due to
 
                  the much longer file names [512 bytes as opposed to
 
                  32 bytes], and larger catalog records (because of
 
                  more/larger fields)).</p>
 
              </TD></TR>
 
            <TR>
 
              <TD>
 
                  <P>Maximum file size</p>
 
              </TD><TD>
 
                  <P>2<SUP>31</SUP> bytes</p>
 
 
              </TD><TD>
 
                  <P>2<SUP>63</SUP> bytes</p>
 
              </TD><TD>
 
                  <P>Obvious user benefit, especially for multimedia
 
                  content creators.</p></td>
 
                  </tr>
 
</table></CENTER>
 
<br>
 
An HFS+ volume contains five special files:
 
<ol>
 
<li>
 
Catalog file - Describes the folder and file hierarchy of the volume. It is organized as a "balanced tree" for fast and efficient searches
 
</li>
 
<li>Extents overflow file - Additional extents (contiguous allocation blocks allocated to forks) are stored in a b-tree in this file
 
</li>
 
<li>
 
Allocation file - Specifies whether an allocation block is free (similar to $Bitmap in NTFS). This is stored in a bitmap, specifying a free allocation block with a "clear bit"
 
</li>
 
<li>Attributes file - Contains attribute information regarding files or folders
 
</li>
 
<li>
 
Startup file - Allows computers to boot that do have built in support for HFS+ file systems
 
</li>
 
</ol>
 
<br>
 
HFS+ also implements journaling, which allows fast recovery in the case of a crash or power outage. According to Apple, "The purpose of the journal is to ensure that when a group of related changes are being made, that either all of those changes are actually made, or none of them are made."[http://developer.apple.com/technotes/tn/tn1150.html#Journal]
 
 
Apple technical notes are available for the HFS+ file system from their [http://developer.apple.com/cgi-bin/search.pl?q=HFS+&num=10&site=default_collection website].
 
 
== External Links ==
 
* [http://web.archive.org/web/20090530120010/http://developer.apple.com/technotes/tn/tn1121.html Internet Archive Copy of: Technical Note TN1121], originally found on http://developer.apple.com/technotes/tn/tn1121.html#HFSPlus
 
 
[[Category:Disk file systems]]
 

Revision as of 19:47, 11 September 2013